PEM (i.e., Proton Exchange Membrane) fuel cells [a.k.a. SPE (Solid Polymer Electrolyte) fuel cells] are well known in the art, and include a "membrane-electrode-assembly" comprising a thin, proton-transmissive, solid polymer, membrane-electrolyte having an anode on one of its faces and a cathode on the opposite face. The membrane-electrode-assembly [a.k.a. MEA] is sandwiched between a pair of electrically conductive elements which (1) serve as current collectors for the anode and cathode, and (2) contain a plurality of flow channels therein for distributing the fuel cell's gaseous reactants, H.sub.2 and O.sub.2 (e.g., air), over the surfaces of the respective anode and cathode catalysts. The flow channel for each reactant is often referred to as a "flow field" for that reactant (e.g., H.sub.2 flow field). A plurality of individual fuel cells are commonly bundled together to form a PEM fuel cell stack, and the stack forms part of a fuel cell system that includes ancillary devices such as reformers, shift reactors, combusters, compressors, humidifiers, fuel storage, pumps and controllers, inter alia.
The solid polymer membranes are typically made from ion exchange resins such as perfluorinated sulfonic acid. One such resin is NAFION.RTM. sold by E.I. DuPont deNemours & Co. Such membranes are well known in the art and are described in U.S. Pat. Nos. 5,272,017 and 3,134,697, and in Journal of Power Sources, Volume 29 (1990) pages 367-387, inter alia. The anode and cathode on the membrane's faces typically comprise finely divided carbon particles, very finely divided catalytic particles supported on the carbon particles, and proton conductive resin intermingled with the catalytic and carbon particles. One such membrane-electrode-assembly and fuel cell is described in U.S. Pat. No. 5,272,017 issued Dec. 21, 1993 and assigned to the assignee of the present invention.
Typically, fuel cell systems are designed so that under normal operating conditions the flow rate of the reactants to the stack will increase as the electrical current demand on the stack increases, and vice-versa. For the cathode (air) this is typically accomplished by increasing or decreasing the system's compressor output in response to the stack's electrical output. For the anode (H.sub.2) this may be accomplished by increasing the pressure regulator on a tank-supplied system, or increasing the fuel supply rate to a reformer in a reformer-supplied system. Likewise under normal operating conditions, both reactant streams are typically humidified, upstream of the stack, to prevent drying of the membrane. In this regard, the reactant streams may either be routed through a membrane or filter-type humidifier, or preferably will have water injected thereinto by means of appropriate injectors. The fuel cell reaction forms water on the cathode side of the membrane.
PEM fuel cell stack performance can degrade for a number of reasons including flooding of the cells with H.sub.2 O. Under normal operating conditions, water will not accumulate in the flow fields, because it is flushed out by the flowing reactant gases. However, sometimes the relative humidity in the reactant streams can exceed 100%, which causes water to condense and form droplets. When these droplets are allowed to build-up over time, the flow fields become partially or totally obstructed (known as "flooding") which prevents (a) the reactants from reaching the reaction sites, and (b) the reaction water from exiting the flow field(s). This, in turn, results in a sharp degradation in stack performance, and requires corrective action.